Submarine groundwater discharge (SGD) is a noted potential mechanism for delivery of certain chemical species to the ocean. SGD involves the upward flow of water into the ocean from the sediments. SGD can bring water and chemicals into salt marshes, coastal waters, bays, coral reefs and other areas. The effect of SGD derived nutrients and pollutants to coastal waters can lead to environmental problems, including eutrophication and the deterioration of the natural ecology. Accordingly, the study of the flow and in particular the volume of the flow of submarine groundwater discharge in saltwater bays is of interest to scientists studying the environment of an estuary or other habitat.
Techniques for measuring groundwater seepage rates are known, and the predominant technique is to employ a drum as a seepage housing and fit the housing with a plastic bag and then secure the housing to the ocean floor. Numerous articles discuss that the volumetric measurement of a seepage rate using a bag on the end of a seepage housing and note that this technique is prone to artifacts (Shaw and Prepas 1989; Belanger and Montgomery 1992; Isiorho and Meyer 1999; Shinn et al. 2002). Specifically, bag-derived flow rates may be biased by constriction of flow by the bag and/or by wave-induced motion of the water inside the bag. Bags, partially prefilled with water prior to deployment, yielded more accurate results than empty bags (Belanger and Montgomery 1992). Moreover, the intensive labor involved in the bag method does not lend itself to time series studies on the scale of lunar tidal cycles and seasons. Perhaps most importantly, the validity of reverse flow measurements using bags has not been adequately proven. For the reasons mentioned above, the development of automated seepage meters have been carried out by several groups, and some of these techniques are discussed in An automated Dye-dilution Based Seepage Meter for the Time-series Measurement of Submarine Groundwater Discharge, Edward Sholkovitz, Craig Herbold, and Matthew Charette Limnol. Oceanogr.: Methods 1, 2003, 16-28 (2003). Several articles are noted therein including one that discusses a heat-pulse based instrument developed by Taniguchi and Fukuo (1993) and Taniguchi and Iwakawa (2001). The timed transmission of heat pulses to downstream thermistors in a flow tube forms the basis of this method. Another seepage meter discussed therein also employs heat-pulse technology (Krupa et al. 1998). Paulsen et al. (2001) have developed an acoustic (ultrasonic) automated seepage meter, based on the timed perturbation of sound in a moving fluid. The paper itself discusses a timed dilution of dye as the basis for calculating the flow. All three instruments use seepage housings to collect and focus the flow through a tube or small chamber. All three instruments employ an “open-system” design that allows unrestricted fluid flow in either direction.
Although the above systems can work well, there exists a need for improved and more accurate flow meter systems that can work in an underwater environment.